1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
Conventionally, an image forming apparatus which adopts a scheme of uniformly charging a photoconductive photoconductive element, “writing an image by means of a light (optical image radiation or optical writing)” on the uniformly charged photoconductive element, forming an electrostatic latent image corresponding to a written image, obtaining a toner image obtained by developing this electrostatic latent image, transferring the obtained toner image to “a sheet recording medium such as a transfer sheet” and fixing the toner image to the sheet recording medium is widely known as a copying machine or a MFP (multi function printer).
In this image forming apparatus, the fixing device “fixes a toner image on, for example, a transfer sheet”, and, if the fixing device does not appropriately function, good image formation is not performed.
Although the toner image to be fixed is a monochrome image, the toner image is usually “a color toner image obtained by mutually overlaying a plurality of toner images of different colors” in a recent image forming apparatus.
Although there are various types of fixing devices, many types which pinch and convey “a sheet recording medium to which a toner image is transferred” by means of a pair of rotating bodies, applies heat and a pressure to the toner image and fuses the toner image to the sheet recording medium. “Pinching and conveying” a sheet recording medium refers to “compression conveying” a sheet recording medium.
Individual rotating bodies generally adopt forms of “rollers or belts”, and a configuration of a pair of rotating bodies can include a combination of rollers, a combination of belts or a combination of a belt and a roller.
One of a pair of rotating bodies which contacts on a surface side of a sheet recording medium, that is, a surface on a side on which a toner image is formed is referred to as a “fixing member”.
The fixing member which is formed in a roller shape is also referred to as a “fixing roller”. Further, the fixing member which is formed in a belt shape is also referred to as a “fixing belt”. The fixing belt is wound around “a plurality of rollers including driving rollers”, and is driven to rotate.
“A direction” in which a surface of the fixing member “moves by way of rotation of the fixing member” corresponds to a direction in which a recording sheet is compression conveyed, and therefore this direction on the surface of the fixing member is referred to as a “conveying direction”.
The surface of the fixing member contacts a toner image to be fixed, and is adhered with “part of toner which forms a toner image” upon fixing. The toner adhered to the fixing member surface in this way is adhered (also referred to as “offset”) to the fixing member surface if left, and attached to a non-image portion on a sheet recording medium surface, thereby causing fixing failure.
To avoid this, a cleaning unit such as a brush is slid against the surface of a sheet recording medium and removes adhered toner in many cases. Further, a peeling claw which peels “a sheet recording medium to which a toner image is fixed” from the fixing member surface or a temperature detecting unit (thermistor) which detects a surface temperature of the fixing member abut on the surface of the fixing member in some cases.
These cleaning unit, the peeling claw and the thermistor physically contact a fixing member surface, and therefore this physical contact “abrades or damages” the surface of the fixing member.
When the surface of the fixing member is abraded or damaged to some degree, the fixing member cannot exercise the function, and therefore this fixing member is exchanged.
Although damage on a fixing member is caused by “contact with the cleaning unit, the peeling claw or the thermistor”, the cause of the damage is not limited to this. There is “a damage resulting from a fixing operation itself”.
Upon fixing, a pair of rotating bodies compression conveys a sheet recording medium, and a surface of a fixing member receives “a reaction of a compression force” from the sheet recording medium.
The sheet recording medium is generally a “transfer sheet”, and is cut into a standard shape such as an A4 size or a B5 size.
An edge portion (a ridge portion between a surface of a transfer sheet and a cutting end surface) of a cutting portion (a portion which forms a thickness of an end portion) of a transfer sheet is sharp, and, upon fixing, a force concentrates on a contact portion contacting a transfer member surface and therefore is likely to produce “a scratch like a slit” on the surface of the fixing member. That the “edge portion” is sharp is understood from the fact that “a finger is cut by an edge of paper” when paper is used in a daily scene.
A scratch is produced by the edge portion of the sheet recording medium like “a streak in the conveying direction” on the surface of the fixing member, and will be referred to as “a streaky scratch” below.
In addition to the above cause, a “streaky scratch” is caused when a transfer sheet is cut into a standard size and “(hard particle) paper additive represented by calcium carbonate” is exposed to a cutting surface.
When the additive exposed to the cutting surface is adhered to the fixing member surface along the cutting plane, a nipping portion which compression conveys a transfer sheet is pressed against the fixing member surface, and a stress concentrates on minute particles, thereby damaging the fixing member surface. Particularly, this is likely to happen when the surface of the fixing member is a hard material.
In case of a plastic sheet for an overhead projector which is frequently used as a sheet recording medium other than a transfer sheet, a “streaky scratch” is also likely to be produced.
Such a “streaky scratch” not only negatively affects the life of a fixing member but also negatively affects image quality.
A case will be described as an example where an image forming apparatus forms images on A3 size and A4 size transfer sheets, and “fixes” these two types of transfer sheets “while conveying” the transfer sheets “in a longitudinal direction”.
In this case, although the width (the length in a direction orthogonal to the conveying direction) of the A4 size transfer sheet to be conveyed is narrower than that of the A3 size, the “streaky scratch” is produced at “a width direction end portion of the A4 size and the width direction end portion of the A3 size” in fixing areas.
When the fixing member in which the “streaky scratch” is produced at the width direction end portion of the A4 size performs fixing with respect to the A3 size transfer sheet, the “streaky scratch” is in a transfer area of the A3 size, a contact pressure of the fixing member and a toner image weakens at a scratch portion, “fixing failure” occurs along the streaky scratch, and a “streaky image abnormality” which is referred to as “picking (a decrease that an image density decreases)” occurs in the fixed image.
In case of a scratch resulting from contact with the above “peeling claw or thermistor”, a similar image abnormality also occurs.
“A fixing member of which surface condition is deteriorated” by “adhered (“offset”) toner”, “a scratch resulting from contact with a peeling claw or a thermistor” or “a streaky scratch” causes fixing failure as described above.
A method of detecting a condition of the “fixed (“offset”) toner” on a fixing member surface or a condition of the above “scratch resulting from contact with the peeling claw or the thermistor” is proposed (Japanese Patent Application Laid-open No. 5-113739).
By contrast with this, a technique of detecting the degree of a scratch (such as a depth or a width of a scratch) of the above “streaky scratch” is not known.
Japanese Patent Application Laid-open No. 2006-251165 (Japanese Patent Application Laid-open No. 2007-34068) discloses a countermeasure of dealing′ with the above “deterioration of a surface condition of a fixing member”.
The method disclosed in Japanese Patent Application Laid-open No. 2006-251165 among the countermeasures disclosed in these Patent Literatures includes optically specifying “information about a surface condition” and controlling an exposing unit in case that “the amount of adhered toner increases” at a portion at which a scratch is produced.
According to this method, no countermeasure is taken for a scratch of the fixing member, and deterioration of the fixing member progresses as is.
The method disclosed in Japanese Patent Application Laid-open No. 2007-34068 includes exposing “a new surface” on a fixing belt surface by having tension rollers which have coarser surfaces than the surface of the fixing belt “constantly slide” against the surface of the fixing member (fixing belt). According to this method, the fixing belt surface is worn away due to constant sliding, and therefore the life of the fixing belt is made shorter more than necessary.
Therefore, there is a need to provide an image forming apparatus which can effectively detect a condition of a fixing member surface, that is, not only the offset and “a scratch resulting from contact with a peeling claw or a thermistor” but also “a streaky scratch produced by contact with the sheet recording medium” in particular.